Spectroscopic studies of doped GaAs/ AlGaAs quantum wells in the high carrier density regime
Sammanfattning: This thesis is mainly based on experimental and theoretical investigations of the electronic structures and optical properties of p-type doped GaAs/AlGaAs quantum wells using a combination of photoluminescence and magneto-optical spectroscopy in the presence of high magnetic fields.The effects of high hole densities up to 3.1 x1012 cm-2 on the optical spectra have been systematically explored for modulation-doped quantum wells. Several novel aspects related to many-body effects have been observed: Firstly, there is a significant red shift observed for the main photoluminescence emission due to band-gap renormalization effect (BGR). Theoretical calculations based on the Hartree self-consistent and the random-phase approximation (RPA) taking into account the finite-well width effect have been established to describe the band-gap renormalization. Secondly, excitons associated with unoccupied bands are found to survive up to a hole density of 2 x 1012 cm-2, i.e. well above the degenerate level. Finally, a novel recombination with light-hole character appears at the Fermi-edge at higher hole concentrations. This transition associated with the light-hole bandtail supported by related theoretical prediction of the luminescence line shape, is the basis for the interpretation of the Fermi-edge emission.In optical spectra in the presence of a magnetic field, magneto-excitons are observed weakly in low doped samples and are replaced by Landau-level transition at high hole concentrations. The Landau-level formation is complicated by the effect of the valence-band mixing and is smeared out at high-hole densities. The predicted Landau-level formations based on self-consistent calculations taking into account the valence-band mixing, the conduction-band nonparabolicity, and the phase-space filling effect have been employed. The agreement achieved for the experimental results and theoretical predictions results in a more accurate determination of band-to-band transition energies and band-gap renormalization effect from different subbands. Theoretical calculations for the multiple-subband renormalization have been subsequently performed. A theoretical model for the multiple-subband renormalization taking into account the inter- and intrasub bandprocesses and the valence-band mixing provides a good agreement with the experimental results.The effect of an AlAs monolayer inserted in n-type highly modulation-doped quantum wells on the optical and transport properties has been investigated. The interband Landau-level transition energies were compared with the results of theoretical calculations based on simple self-consistent calculations. Finally, studies of an acceptor confined in a quantum well have been presented. Satellite spectra associated with an acceptor bound exciton were observed. The satellite spectra also in the presence of a magnetic field have been reported, the experimental results have been compared with theoretical calculations and provided essential information for the electronic structure of the confined acceptor.
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